Group-III nitride semiconductors, which are compound semiconductors typified by GaN, have a wide band gap, and therefore, they are widely used as materials for light-emitting devices, such as blue, green, and other color LEDs (light-emitting diodes), LDs (laser diodes), and the like, and power devices. Silicon, which typifies semiconductor materials, is generally used as a wafer having a large diameter that is obtained by cutting from a bulk crystal with a large diameter. However, for such compound semiconductors as mentioned above, it is extremely difficult to obtain a bulk crystal having a large diameter (for example, 4-inch dia or larger). Therefore, in manufacturing a semiconductor device using such a compound semiconductor, a wafer in which the compound semiconductor is heteroepitaxially grown on a substrate formed of a material dissimilar thereto is generally used. In addition, a p-n junction or a heterojunction which constitutes an LED or an LD can also be obtained by further carrying out an epitaxial growth thereon.
For example, as a material of an epitaxial growth substrate on which a GaN single crystal can be grown, sapphire, and the like, are known. With sapphire, a bulk single crystal having a large diameter can be relatively easily obtained, and by selecting the plane orientation therefor as appropriate, a GaN single crystal can be heteroepitaxially grown on a substrate made of a single crystal of sapphire. Thereby a wafer having a large diameter in which a GaN single crystal has been formed can be obtained.
Here, with a p-type GaN layer and an n-type GaN layer being formed on a sapphire substrate, a pn junction is formed, however, generally, it is difficult to obtain a good-quality p-type GaN layer, as compared to obtain an n-type GaN layer. Therefore, for this structure, a thick n-type GaN layer is generally formed on a sapphire substrate, and on the n-type GaN layer, a thin p-type GaN layer is formed by epitaxial growth in sequence. With this structure, since sapphire for use as the substrate is nonconductive, electrical contacts to the p-type GaN layer and the n-type GaN layer are often provided on the top side (on the side opposite to the substrate). Sapphire is transparent, and therefore, with a light-emitting device, luminescence can be taken out from the bottom side thereof (which structure is known as a flip chip structure).
FIG. 9(A) and FIG. 9(B) show a simplified manufacturing process for a light-emitting device having such structure. With this manufacturing method, as shown in FIG. 9(A), first, an n-type GaN layer 92 and a p-type GaN layer 93 are formed on a sapphire substrate 91 in sequence. Actually, between the n-type GaN layer 92 and the sapphire substrate 91, a buffer layer is often formed in order to improve the crystallinity of the n-type GaN layer 92, however, description of the buffer layer is omitted here. Thereafter, as shown in FIG. 9(B), the surface of the p-type GaN layer 93 is partially etched away to thereby form a region where the n-type GaN layer 92 is exposed, and in this portion, an n-side electrode 94 is formed, while, on the surface of the p-type GaN layer 93, a p-side electrode 95 is formed.
The material constitution of the electrode in such a structure is disclosed in, for example, Patent Document 1. In this document, it is disclosed that a structure in which, especially as a layer in the n-side electrode 94 that is to be contacted with the n-type GaN layer 92, a Cr or Cr alloy layer is formed by sputtering, and thereon an Au layer is formed through a Ti layer has a good ohmic contact characteristic on the n-type GaN layer 92. In addition, in Patent Document 2, it is disclosed that an alloy of Ti and Al has a good ohmic contact characteristic on the n-type GaN layer 92. In other words, by connecting an electrode having such structure to the n-type GaN layer 92, the electrode resistance can be lowered, and a light-emitting device having a good luminescent property can be obtained.
With the structure in FIG. 9(B), luminescence can be taken out from the bottom side, however, the region where the top side of the n-type GaN layer 92 is exposed as shown at right in FIG. 9(B) will not utterly contribute to the luminescence. Therefore, as a form which provides a higher luminescence efficiency, a structure in which the sapphire substrate used as the growth substrate is removed, and on the back side of the n-type GaN layer, the n-side electrode is formed has been adopted. FIGS. 10(A) to 10(C) illustrate a simplified manufacturing method for a light-emitting device having such structure.
With this manufacturing method, as shown in FIG. 10(A), an n-type GaN layer 92 and a p-type GaN layer 93 are first formed in sequence on the sapphire substrate 91 through a lift-off layer 96. Thereafter, as shown in FIG. 10(B), the lift-off layer 96 is removed by making a chemical treatment (chemical lift-off) or illuminating laser light (laser lift-off). Thereby, the sapphire substrate 91 and the n-type GaN layer 92 are separated from each other, resulting in the bottom face of the n-type GaN layer 92 being exposed. Thereby, as shown in FIG. 10(C), the n-side electrode 94 can be formed in a portion of the bottom face of the n-type GaN layer 92, and the p-side electrode 95 can be formed on the top face of the p-type GaN layer 93. This structure can provide a large effective light-emitting area as compared to that obtained with the structure in FIG. 9(B), thereby a higher luminescence efficiency can be achieved. Further, since there is no need for taking out light from the top face of the p-type GaN layer 93, it is also possible that the area of the p-side electrode 95, which is not transparent to the light, is increased to thereby form the p-side electrode 95 over a wider range within the surface of the p-type GaN layer 93. Generally, since the p-type GaN layer 93 has a high electrical resistivity as compared to that of the n-type GaN layer 92, increasing the area of the p-side electrode 95 is effective for reducing the resistance of the electrode portion. Further, if, for the p-type ohmic electrode, which is to be contacted with the p-type GaN layer, a material having a high reflectivity for the luminescence wavelength is used, the light from the light-emitting layer can be reflected to the opposed face side, whereby a still higher luminescence efficiency can be obtained.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2005-197670    Patent Document 2: Japanese Unexamined Patent Application Publication. No. Hei 7-45867